1. Field of the Invention
The invention relates to an image reading apparatus such as copying apparatus, image scanner, or the like using a contact type image sensor.
2. Related Background Art
FIG. 4A shows an image scanner as an example of an image reading apparatus. FIG. 4B is a cross sectional view of a main portion of the image scanner. In FIG. 4A or 4B, reference numeral 1 denotes an original plate glass; 2 a pressing plate to press an original 20 put on the original plate glass 1; 3 a contact type image sensor; 4 a frame of the image sensor 3; 5 a lens array; 6 a sensor; 7 a light guide; 8 a sensor holder; 9 a guide shaft; 10 a wire winding barrel (drum); 11 a wire for driving the image sensor; and 12 a driving motor.
Generally, an LED is used as a light source (not shown). The LED is arranged in an edge portion of the light guide 7. The light beam emitted from the LED is converted into a linear light beam by the light guide 7 to irradiate the surface of the original. An image of the original 20 on the original plate glass 1 is formed onto the surface of the sensor 6 by the lens array 5 and photoelectrically converted by the sensor 6. By forming the above construction as one unit, the contact type image sensor 3 is formed.
The contact type image sensor 3 is held by the sensor holder 8 which is a holding member and linearly moves in the direction (scanning direction shown by an arrow) perpendicular to the longitudinal direction of the contact type image sensor 3 along the guide shaft 9 while keeping a distance from the original plate glass 1, thereby scanning the plane-shaped original.
Generally, the frame 4 of the contact type image sensor 3 is made of a resin. The contact type image sensor 3 is formed by fixing various component parts such as light guide 7, lens array 5, sensor board, and the like onto the frame 4. The frame 4 of the contact type image sensor 3 generally has a rod shape in terms of a construction of each of the above parts. Therefore, when it is formed, a tendency of xe2x80x9cwarpxe2x80x9d is likely to occur. FIG. 5 shows an enlarged diagram of the main portion of such a state.
When the frame 4 has such a warp tendency, a board of the lens array 5 and sensor 6 fixed onto the frame 4 is also deformed in a frame shape. The original plate glass 1 is usually formed by cutting a plate glass called a xe2x80x9cfloat glassxe2x80x9d into a desired shape and used. The glass ordinarily has precision that is extremely close to that of a flat surface, so that a radius of curvature of the glass is equal to 100 to 500 mR.
In this case, as shown in FIG. 5, a distance between the original plate glass 1 and contact type image sensor 3 differs depending on the location in the longitudinal direction of the contact type image sensor 3. Therefore, although an object surface focal position of the lens array 5 and the upper surface of the original plate glass 1 coincide in the edge portion of the contact type image sensor 3, the object surface focal position of the lens array 5 is further deviated to the upper side than the upper surface of the original plate glass 1 in the center portion of the contact type image sensor 3 due to an influence by a warp of the sensor.
It is now assumed that a distance from the top edge surface of the lens array 5 to the object surface side focal position is set to d, a distance from the top edge surface of the lens array 5 to the upper surface of the original plate glass 1 is set to dxe2x80x2, and a depth of field of the lens is set to D. In this case, the following relational expression has to be satisfied to avoid the occurrence of an out-of-focus of the original image to be read.
dxe2x88x92D/2xe2x89xa6dxe2x80x2xe2x89xa6d+D/2
If distance dxe2x80x2 from the top edge surface of the lens array 5 to the upper surface of the original plate glass 1 lies within the range obtained by the above expression, since the upper surface of the original plate glass 1 is located in the depth of focus of the lens array 5, an out-of-focus does not occur. However, if a value of dxe2x80x2 exceeds the range obtained by the above expression and is shorter than (dxe2x88x92D/2) or longer than (d+D/2), there is a problem such that an out-of-focus occurs. Particularly, in recent years, a conjugate length of the lens array which is used in the contact type image sensor becomes short in accordance with a demand for miniaturization of the apparatus, so that the depth of field is shallow. Consequently, the problem of the out-of-focus is likely to occur.
Generally, a size of gap between the original plate glass 1 and contact type image sensor 3 is very small and there is also a problem such that if a warp amount of the contact type image sensor 3 is large, the lens array 5 or frame 4 is come into contact with the back surface of the original plate glass 1 and an abnormal sound is generated.
FIG. 6 is a schematic diagram of a frame die to form the frame 4 of the contact type image sensor. The direction of the warp of the sensor frame 4 can be controlled by a frame shape or a gate position or by providing a temperature difference between a cabinet side 16 of the frame die to form the frame and a core side 15, or the like. For example, if a temperature of the die on the cabinet side is set to be higher than that on the core side, a contraction on the core side of a low frame temperature is large, so that the frame can be controlled to a shape that is warped upward. It is, however, substantially difficult to suppress a warp amount to zero.
As means for correcting the warp of the sensor frame 4, there is a method of fixing the contact type image sensor 3 to a member such as an iron plate or the like and keeping the flatness by using the flatness and rigidity of the iron plate. There is, however, a problem such that an excessive force is applied to the contact type image sensor 3, longevity and optical characteristics are influenced, and the like. Since the member such as an iron plate is used, there is also a problem of costs such that the number of parts increases.
In an image reading apparatus using a contact type image sensor, it is an object of the invention to prevent a deterioration of a picture quality such as out-of-focus or the like due to a warp of the contact type image sensor and a contact between the contact type image sensor and an original plate glass.
According to one aspect of the invention, there is provided an image reading apparatus comprising: an original plate glass to put an original; a contact type image sensor to read image information of the original; and moving means for moving the contact type image sensor relatively to the original, wherein a warp shape of the original plate glass in the longitudinal direction of the contact type image sensor and a warp shape of the contact type image sensor are almost the same.
By constructing as mentioned above, the deterioration in picture quality such as out-of-focus or the like and the contact between the contact type image sensor and the original plate glass can be prevented.